This invention generally relates to a control circuit for selectively varying the number of character text rows simultaneously displayed on the screen of a cathode ray tube.
A variety of video systems for alphanumeric character display have been suggested and implemented. One approach has been the generation of a dot matrix by means of a plurality of vertically spaced horizontal scans of a selectively blanked electron beam directed at the screen of a cathode ray tube. During scanning, the beam is pulsed to produce the desired dot pattern for generating the selected characters. A particular arrangement for producing such a matrix is described in U.S. Pat. No. 3,868,673 issued to T. J. Mau et al. on Feb. 25, 1975, entitled "Display Apparatus Including Character Enhancement." The raster produced on the cathode ray tube (CRT) display screen may conceptually be divided into several horizontal text rows each including a plurality of sequential scan lines, e.g., fourteen. Additionally, each text row may be divided into a plurality of character areas. Frequently, space is provided for sixty four or eighty characters on each text row. The cathode ray tube electron beam is pulsed during each scan to successively write a portion of each of the characters in the selected text row. Heretofore, the overall frequency relationships of the system have been selected so that each text row has the same number of scan lines and an equal number of scan lines are utilized during the vertical retrace interval. A typical arrangement utilizes twenty-four text rows consisting of fourteen scans each and an additional group of fourteen scans is utilized during vertical retrace of the CRT beam. The total number of scan lines being three hundred fifty (25.times.14). Such a system is described in the previously cited U.S. patent wherein the height of each character is nine scan lines with the remaining five scan lines in each text row providing spacing between contiguous text rows.
Frequently, it is desirable to provide the operator with an advisory service message without significantly disrupting the working text display. Heretofore, the presentation of such an advisory message has required switching the entire screen format from the working text to the advisory message. The illustrated embodiment provides a means for compressing the working text display so as to provide two additional text rows for the display of additional information such as an operator service message. As will be subsequently more fully appreciated, the additional text rows are obtained by reducing the number of scans per text row without reducing in the character size, that is, by reducing the spacing between contiguous character rows.